Hollow fiber modules with hollow fibers made in a U shape--also called hairpin-shaped--are known in the art, for example as so-called dead-end filter candles, the microporous wall of the hollow fibers functioning as the actual filter agent.
Hollow fibers with a thin wall or with semipermeable or selective properties are as a rule also described as hollow fiber membranes, membranes in hollow fiber form, as capillary membranes in the case of small diameters and in similar terms. Such membranes having the last mentioned properties are also described as nonporous or dense membranes.
In hollow fibers, the separation (filtering) can take place from the outside inwards, that is to say into the interior (lumen) of the hollow fibers, but also in the converse direction. All dead-end separation devices (filters) of this type here have the common feature that the filtrate or permeate can be continuously discharged until the filter agent is exhausted, while the part or parts of the starting medium (non-filtrate), which has or have been separated off (filtered off), remain(s) in the outer space surrounding the hollow fibers or in the hollow fibers, and is or are sometimes deposited on the filter surface and sometimes form(s) a filter layer (filter cake) and is or are not likewise discharged continuously as, for example, in crossflow filtration. This is the reason for the name dead-end.
It has proved to be a disadvantage in the known hollow fiber modules with hollow fibers made in a U shape that the hollow fiber bundle takes up a substantially greater volume in the region of the substantially semicircular bend of the hollow fibers than in the region of the embedding of the end regions of the hollow fibers. Depending on the number of hollow fibers, such hollow fiber bundles therefore have a very pronounced pear shape. This has the consequence that the housing in which such a hollow fiber module is to be installed must have substantially greater dimensions, and in the case of a round crossection also a substantially greater diameter than the dimensions (the diameter) of the sealing compound body would require. This means that the space requirement of the known hollow fiber modules with hollow fibers made in a U shape is relatively large, which amounts to a not inconsiderable part of the costs in large-scale plants with a multiplicity of such hollow fiber modules.